1. Field of the Invention
The present invention relates to nitrided maraging steel which can be used as a metal belt in a belt-type variable speed drive for the car, etc., for example, and a method of manufacturing the same.
2. Description of the Related Art
The metal belt employed in the belt-type variable speed drive has a structure as shown in FIG. 1. For example, about 400 pieces of metal blocks 10 are coupled in an endless fashion by two sets of metal endless rings 20 each of which is constructed by laminating the endless rings up to about nine layers. The endless rings 20 need the high tensile strength that is required for the power transmission and the fatigue life that withstand the repeat bending stress caused when the belt is wound on the pulley. For this reason, the maraging steel that is known as the ultra high strength steel is employed as the endless rings 20. In addition, the nitriding is applied to this maraging steel to improve the fatigue life.
However, the passive oxide film due to the oxidation is formed on the surface of the steel that contains a number of alloy elements, like the maraging steel. Therefore, the permeation of the nitrogen is prevented by the passive film and thus the uniform nitriding become difficult.
Up to now, as the nitriding method for the maraging steel, there have been proposed the method of executing the nitriding after the oxide film is removed by using the hydrogen chloride or the phosphoric acid, the method of executing the nitriding in the nitriding atmosphere to which the hydrogen sulfide is added, the method of executing the nitriding after the oxide film on the surface is reduced by using the fluorine compound, the method of executing the nitriding in the salt bath, etc. so as to improve the fatigue life.
However, the problem such as the stability in quality, the workability, the economical efficiency, the measure for the environmental pollution, or the like still remains in the above nitriding methods. For example, the method of removing the passive oxide film by using the hydrogen chloride or the phosphoric acid fails to provide a sufficiently uniform nitrided case. Also, since the method of executing the sulphonitriding in the nitriding atmosphere, to which the hydrogen sulfide is added, employs the hydrogen sulfide having the strong toxicity, the heavy burden in the handling and management of the chemicals is imposed. Further, the method of executing the nitriding after reducing the oxide film on the surface by using the fluorine compound needs the high process cost, because the nitriding must be executed after the fluoriding treatment. In addition, the fluorine compound remains on the surface of the maraging steel. Also, since the method of executing the nitriding in the salt bath employs the cyanogens, the measure for the environmental pollution such as the waste water process is needed and thus there is the problem in the workability.
Further, since the endless ring for the metal belt that is constructed by laminating about ten sheets of rings, as shown in FIG. 1, is used, variation in the elongation of the circumferential length of the rings caused before and after the nitriding must be reduced. Therefore, it may be considered that, like the nitriding method disclosed in Japanese Patent Application Publication Sho 59-80772 (published in 1984), the nitrided depth must be formed shallow by lowering the ammonia concentration upon the gas nitriding to reduce the permeation of the nitrogen.
However, in recent years, the needs for the higher output of the engine and the reduction in size and weight are enhanced. Thus, the higher strength of the metal belt is desired in order to satisfy such request. Accordingly, in order to achieve the higher strength of the maraging steel, it is requested to form the uniform and deeper nitrided case by improving much more the nitriding in the prior art.
Also, since the maraging steel is the precipitation hardening alloy steel, the age hardening makes progress simultaneously with the nitriding to change the mechanical characteristics. For this reason, the selection of the nitriding treating profiles to sufficiently bring out the characteristics of the maraging steel, that is neither insufficiently aged nor excessively aged, must be checked.
It is an object of the present invention to provide a maraging steel that is excellent in fatigue life and has a small amount of variation in the length based on the nitriding, and an endless ring for a metal belt in a variable speed drive using the same.
It is another object of the present invention to improve the stability in quality, the workability, the economical efficiency, etc. in the nitriding of the maraging steel, and also to prolong the fatigue life by improving the fatigue life.
A first aspect of the present invention provides a maraging steel which comprises a plate which is subjected to an aging to have a hardness of more than 500 Hv; and a nitrided case formed on a surface layer of the plate. The nitrided case has a depth that is about 20% to 50% to a xc2xd thickness of the plate.
A second aspect of the present invention provides an endless ring applied to a metal belt. This endless ring comprises a maraging steel plate which is subjected to an aging to have a hardness of more than 500 Hv; and a nitrided case formed on a surface layer of the plate. This nitrided case has a depth that is about 20% to 50% to a xc2xd thickness of the plate.
A third aspect of the present invention provides a method of manufacturing the above maraging steel plate. This method comprises executing an aging to have a hardness of more than 500 Hv; and forming a nitrided case whose depth is about 20% to 50% to a distance from a plate surface to a center. This nitrided case is formed by executing a nitriding in a nitrogen atmosphere, to which organic chloride is added, after the aging.
A forth aspect of the present invention provides a method of manufacturing the above endless ring applied to the metal belt. This method comprises executing an aging to have a hardness of more than 500 Hv; and forming a nitrided case whose depth is about 20% to 50% to a distance from a plate surface to a center. The nitrided case is formed by executing a nitriding in a nitrogen atmosphere, to which organic chloride is added, after the aging.